1. Field of the Invention
The present invention relates to a ferroelectric memory (FRAM), and more particularly, to a ferroelectric memory having an electromagnetic wave shield structure so as not to receive the effects of electromagnetic waves from outside when mounted in an IC card or the like.
2. Description of the Related Art
Ferroelectric memory makes use of the phenomenon by which, when a ferroelectric material is polarized by the application of an oriented electric field, the state of polarization thereof remains as residual polarization even when the power source is shut OFF. Such ferroelectric material can be polarized using a relatively smaller amount of energy than an electrically erasable programmable read-only memory (EEPROM) or the like, and ferroelectric memory can write, erase and read data in a shorter period of time than EEPROM or flash memory. Moreover, because ferroelectric memory is capable of retaining stored data even after power has been disconnected, it is attracting attention as a next-generation non-volatile memory. For example, ferroelectric memory is being studied for use as rewritable program ROM, and as a replacement for conventional dynamic random access memory (DRAM).
As a useful application of ferroelectric memory, it has been proposed that ferroelectric memory be mounted in integrated circuit (IC) cards, which retain large amounts of data, and have a variety of functions. Such an IC card is normally connected to an external computer in a non-contact state, and is supplied with power, and transmits and receives communications data through the use of electromagnetic waves. For this reason, an antenna for transmitting and receiving electromagnetic waves, which is connected to a ferroelectric memory chip, is provided inside an IC card.
However, to be supplied with power and to transceive data without making contact, a ferroelectric memory-mounted IC card is exposed to an electromagnetic wave of extremely high energy. This electromagnetic wave is irradiated on the built-in ferroelectric memory chip as well as the antenna inside the IC card. Inside a ferroelectric memory chip are provided a word line, a plate line, and a bit line, which is perpendicular thereto, and a memory cell having a ferroelectric capacitor is formed at the intersection thereof. In this case, because either the word line and plate line, or the bit line are laid out within the chip having a relatively long distance, the electric potential thereof can be expected to fluctuate upon the application of a high frequency voltage in accordance with an electromagnetic wave. As a result of the fluctuation of the potential of these lines, there are cases in which an electric field, the direction of which differs from that at write time, is applied to a ferroelectric capacitor connected to a plate line, and the state of stored residual polarization is either reversed or destroyed.
Accordingly, an object of the present invention is to provide a ferroelectric memory, which is constituted so that there is no fluctuation of a storage state resulting from an electromagnetic wave.
Another object of the present invention is to provide a ferroelectric memory, which is constituted so that an electromagnetic wave is not irradiated onto a ferroelectric capacitor that is storing data.
To realize the above-mentioned objects, the present invention is a ferroelectric memory chip having a memory cell region in which there is provided a plurality of memory cells, each having a ferroelectric capacitor, this ferroelectric memory chip being characterized in that there is formed an electromagnetic wave shield layer, which shields the above-mentioned memory cell region against electromagnetic waves from the outside. The electromagnetic wave shield layer is constituted, for example, from either a conductive layer, or a semiconductor layer, which is provided above and/or below the memory cell region, and preferably is connected so as to constitute the same electric potential. Providing such an electromagnetic wave shield layer eliminates the direct irradiation of electromagnetic waves on a word line, plate line and bit line inside the memory cell region, thus making it possible to prevent a change in a storage state by an unexpected electric field being applied to a ferroelectric capacitor inside a memory cell.
To realize the above-mentioned objects, the present invention is a ferroelectric memory chip having a memory cell region in which there is provided a plurality of memory cells, each having a ferroelectric capacitor, this ferroelectric memory chip comprising an electromagnetic wave shield layer, which shields the above-mentioned memory cell region against electromagnetic waves from the outside.
According to the preferred embodiment of the present invention, the above-mentioned electromagnetic wave shield layer has a line-shaped shield line, which is above at least one wire of a plurality of word lines, bit lines, and plate lines connected to each memory cell and formed in the memory cell region, and which is aligned with these lines.
The ferroelectric material of a ferroelectric capacitor is generally an oxide. Therefore, it is desirable that the generation of reduction gases be suppressed as much as possible in fabrication processes subsequent to the formation of a ferroelectric capacitor. In this case, by making the electromagnetic wave shield layer the above line shaped shield line that is aligned with a bit line or other wire as described hereinabove, it is possible to make the surface area of the electromagnetic wave shield layer as small as possible. As a result thereof, it is considered that the generation of hydrogen and other reduction gases generated during the formation of an electromagnetic wave shield layer can be reduced, and that it will be possible to put an end to generated reduction gases residing in the vicinity of a ferroelectric capacitor.
For an IC card, which is mounted with the above mentioned ferroelectric memory chip, and in which is formed an electromagnetic wave antenna for supplying power to, and transmitting/receiving data to/from this chip, it will be possible to prevent, in accordance with an in-chip electromagnetic wave shield layer, any effects to the memory cell region even when subjected to the irradiation of electromagnetic waves from outside.